Purifying tungsten



@atented Feb. 193$ entree stares entrant orator;

ES FRE DERICK LIAM BATES, 0F CLEVELAND HEIGHTS, OHIO, ASSIGNOB T0 TEEQHIO INSERUIKENT MANUFACTUBIH'G COMPANY, OF CLEVELAND, OHIO, A.

PURIFYING TUNGS'JEEN Ito Drawing.

This invention relates to the production and purification of tungsten,and to electric conducting elements made therefrom, especially thosewhich become highly heated in use such as electrical make-and-breakcontact points, incandescent filaments, etc. In the production oftungsten articles the customary procedure is first to reduce tungsticoxide by hydrogen with the production or" a granular metallic powder,and next to sin ter the resulting granular substance together into theform of a somewhat porous rod or ingot by the use of a very high temperature, after which the same is wrought by swaging, drawing, etc., theprocedure being substantially the same as was employed in the making ofwrought iron in the days when a sufficiently high temperature for thecomplete fusion of the iron was unattainable. l have discovered,however, that in the reduction of these oxides by hydrogen it is sodiliicult as practically to be impossible to obtain a completeelimination of the oxygen, and the remaining oxygen, though only a smallportion of that present originally in the oxide, seriously impedes thesubsequent manipulation and use of the metal by interfering with theproper welding of the difierout parts together, by rendering the articlebrittle and non-homogeneous, by tending to produce offsetting or saggingin wires, and pitting in the spark points, and otherwise misbehaving.

l have discovered, however, that it is possible to obtain a completeelimination of the oxygen, with the avoidance of the disadvantages abovementioned, by impregnating the article at the outset with a morevolatile metal, provided this metal be one whose combination with oxygenis either unstable or sufficiently volatile to be dissipated by thetemperatures employed.

Such a metal is silver; and it is within my invention to mix finelydivided silver with the previously reduced granular tungsten, andafterwards to compact and sinter the whole, finally raising thetemperature to such a degree as wholly to volatilize the silver whichtakes with it all traces of oxygen. The resulting bar or ingot shows notrace Application filed November 5, 1928. Serial No. 317,493.

of silver but is more easily worked than when made in the usual manner,is more ductile, and shows less sagging or ofi'setting when made intowire.

Another way of securing the intimate admixture of metals necessary tothe performance of this process is to reduce them simultaneously byhydrogen. One way in which this can be done is to mix silver chlorideintimately with the tungsten or other oxide, prior to the reduction byhydrogen, since silven chloride is likewise reduced under theseconditions. A still better way is to precipitate the silver compoundinto the tungstic oxide so as to obtainthe most complete incorporationat all stages of the process. For example in the production of tungsten,it is customary to precipitate sodium tungstate solution with dilutehydrochloric acid according to the formula NaQWO, 2HO1=2NaCl +WO B 0 Toproduce the desired silver salt it is only necessary to add the properproportion of silver nitrate solution to one of the original solutions,it makes little dillerence which. If for example the silver nitratesolution and sodium tungstate solution be added simultaneously to dilutehydrochloric acid a mix ture of the two precipitates will occur. Anotherway is to add the silver nitrate solution to the sodium tungstatesolution thus producing a brown, cloudy, precipitate of silver oxidewhich is converted by the hydrochloric acid into silver chloride.

No added precautions are necessary in the reduction or subsequenttreatment of the metal, the steps of washing and drying the precipitate,reduction by hydrogen, pressing, sintering, swaging, and drawing beingperformed in the usual manner. At one temperature in the operation thesintercd bar will be found more or less covered with drops of moltensilver driven out of the pores by the heat, but it is necessary for theproper welding of the tungsten particles together that the temperaturebe raised to such a degree that all of this silver is volatilized.However, the subsequent processes are much facilitated, the ductilityand strength of the metal is increased, and the sagging of filaments islargely overcome.

When such tungsten is made into spark points, make-and-break devices, orarcing terminals its tendency to disintegration and pitting is muchdecreased, and this without the retention of any substances inside thearticle which increase its electricalresistance or tend to impede theformation of a clean contact.

Another advantage of this treatment is' found in the case of thoriatedfilaments. It is well known that the usual tungsten filament cannot beworked successfully with a thoria content appreciably above 1.5%although an increase in the thoria content enhances the emissivity. Bythe employment of silver as above described I have found it possible toincrease the thoria content up to as much as 2 or 3% with correspondingimprovement in' the properties mentioned, and without increasing thebrittleness or hardness suiiiciently to preclude successful working. Ieffect this by an extension of the same process, namely by adding thedesired amount of thorium nitrate solution to the mixture of tungsticoxide and silver chloride prior to the reduction by hydrogen. Thisreduction serves merely to change the thorium nitrate to oxide and Ifind that with the presence of silver as herein described I can work theresulting metal into filament form successfully even with a thoriacontent up to 3% although the silver is wholly volatilized during thesintering operation. This advantage is present whether or not specialadditional steps are taken to reduce the thoria to metallic form.

In the plain reduction and purification of tungsten for spark point orfilament use I generally use an amount of silver equal to about 5% to10% of the tungsten. There is no apparent advantage in using more, nordisadvantage except the added cost. In making thoriated filaments,however, I have found it desirable to increase the silverin proportionto the amount of thoria contained so as to keep the amount of silverabout equal to that of the thoria. It has been suggested and certainobserved facts appear to confirm the suggestion that the effect of thesilver is in some way to reduce the thoria to metallic form, andcertainly the physical condition of the resulting mass is notablyaltered by its use, its improved grain structure bein compacting thesame, sintering at atempera-.

ture sufiiciently highto volatilize all the silver, and afterwardswaging and drawing.

In testimony whereof I hereunto affix my signature.

(IHARLES FREDERICK WILLIAM BATES.

visible even to the naked eye, but as to this I offer no theory butstate merely the result of actual experience.

Having thus described my invention what I claim is:

1. Process of producing tungsten articles which are free from includedtungstic oxide

